lkml.org 
[lkml]   [2015]   [Mar]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [BUG/PATCH] kernel RNG and its secrets
On 03/18/2015 11:50 AM, Hannes Frederic Sowa wrote:
>
>
> On Wed, Mar 18, 2015, at 10:53, mancha wrote:
>> Hi.
>>
>> The kernel RNG introduced memzero_explicit in d4c5efdb9777 to protect
>> memory cleansing against things like dead store optimization:
>>
>> void memzero_explicit(void *s, size_t count)
>> {
>> memset(s, 0, count);
>> OPTIMIZER_HIDE_VAR(s);
>> }
>>
>> OPTIMIZER_HIDE_VAR, introduced in fe8c8a126806 to protect crypto_memneq
>> against timing analysis, is defined when using gcc as:
>>
>> #define OPTIMIZER_HIDE_VAR(var) __asm__ ("" : "=r" (var) : "0" (var))
>>
>> My tests with gcc 4.8.2 on x86 find it insufficient to prevent gcc from
>> optimizing out memset (i.e. secrets remain in memory).
>>
>> Two things that do work:
>>
>> __asm__ __volatile__ ("" : "=r" (var) : "0" (var))
>
> You are correct, volatile signature should be added to
> OPTIMIZER_HIDE_VAR. Because we use an output variable "=r", gcc is
> allowed to check if it is needed and may remove the asm statement.
> Another option would be to just use var as an input variable - asm
> blocks without output variables are always considered being volatile by
> gcc.
>
> Can you send a patch?
>
> I don't think it is security critical, as Daniel pointed out, the call
> will happen because the function is an external call to the crypto
> functions, thus the compiler has to flush memory on return.

Just had a look.

$ gdb vmlinux
(gdb) disassemble memzero_explicit
Dump of assembler code for function memzero_explicit:
0xffffffff813a18b0 <+0>: push %rbp
0xffffffff813a18b1 <+1>: mov %rsi,%rdx
0xffffffff813a18b4 <+4>: xor %esi,%esi
0xffffffff813a18b6 <+6>: mov %rsp,%rbp
0xffffffff813a18b9 <+9>: callq 0xffffffff813a7120 <memset>
0xffffffff813a18be <+14>: pop %rbp
0xffffffff813a18bf <+15>: retq
End of assembler dump.

(gdb) disassemble extract_entropy
[...]
0xffffffff814a5000 <+304>: sub %r15,%rbx
0xffffffff814a5003 <+307>: jne 0xffffffff814a4f80 <extract_entropy+176>
0xffffffff814a5009 <+313>: mov %r12,%rdi
0xffffffff814a500c <+316>: mov $0xa,%esi
0xffffffff814a5011 <+321>: callq 0xffffffff813a18b0 <memzero_explicit>
0xffffffff814a5016 <+326>: mov -0x48(%rbp),%rax
[...]

I would be fine with __volatile__.

Thanks a lot mancha, could you send a patch?

Best,
Daniel


\
 
 \ /
  Last update: 2015-03-18 12:21    [W:0.076 / U:0.212 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site